Bug report
Bug description:
The HAMT held by a context is immutable, but the pointer to it is not: contextvar_set() and contextvar_del() install a new HAMT and drop the reference to the previous one. Only the thread that entered a context can do that (a context cannot be entered by two threads at once), but any thread can read a context object at any time -- ctx.copy(), len(ctx), ctx.items(), etc. Such a reader has to acquire its own reference to the HAMT under the context's lock; reading ctx_vars unlocked lets the writer deallocate the HAMT while the reader is walking it.
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux